Prior art which is related to this technical field can e.g. be found by the technical specifications TS 36.213 (current version: 8.6.0) of the 3GPP.
The following meanings for the abbreviations used in this specification apply:
3GPP: Third Generation Partnership Project
AGG: Aggregation Level on PDCCH
AIF: Air Interface
AMC: Adaptive Modulation and Coding
ARQ: Automatic Repeat Request
CCE: Channel Coding Element
CDF: Cumulative Distribution Function
CL: Closed Loop
CQI: Channel Quality Indicator
CSS: Common Search Space
CW: Codeword
DCI: Downlink Control Information
DIV: Diversity
DL: Downlink
DRX: Discontinuous Reception
eNB: Evolved Node B (eNodeB)
EPC: Evolved Packet Core
EPS: Evolved Packet System
E-UTRAN: Evolved UTRAN
LTE: Long Term Evolution
MCS: Modulation and Coding Scheme
MIMO: Multiple-In-Multiple-Out Antenna System
MoRSE: Mobile Radio Simulation Environment
OLLA: Outer Loop Link Adaptation
PC: Power Control
PCFICH: Physical Control Format Indicator Channel
PDCCH: Physical Downlink Control Channel
PDSCH: Physical Downlink Shared Channel
PHICH: Physical Hybrid ARQ Indicator Channel
PUSCH: Physical Uplink Shared Channel
PRB: Physical Resource Block
PSD: Power Spectral Density
OFDM: Orthogonal Frequency Division Multiplex
OL: Open Loop
QAM: Quadrature Amplitude Modulation
QPSK: Quadrature Phase Shift Keying
RACH: Random Access Channel
RAN: Radio Access Network
RNTI: Radio Network Temporary Identifier
RRC: Radio Resource Control (Signaling)
SC-FDMA: Single Carrier Frequency Division Multiple Access
SM: Spatial Multiplexing
SNIR: Signal-to Noise Ratio
TTI: Transmission Time Interval
TU3: Channel Model Typical Urban 3 km/h
UE: User Equipment
UL: Uplink
UMTS: Universal Mobile Telecommunications System
USS: UE Specific Search Space
UTRAN: UMTS Terrestrial Radio Access Network
Recently, the 3GPP LTE technology is under study for introduction as successor of 3G UMTS providing wireless broadband access with high capacity, high user data rates and low latency/access times.
According to LTE, PDCCH is the downlink control channel, which carries the information about the structure and allocation of the shared traffic channels in UL and DL (PDSCH, PUSCH), i.e. the PDCCH contains the UL and DL grants for physical resource block (PRB) allocation, modulation and coding scheme (MCS) selection as well as power control commands, current MIMO mode etc. which are to be submitted e.g. by an evolved Node B to scheduled UEs in a certain TTI. In addition, the PDCCH also covers signaling allocation such as e.g. broadcast, paging and random access response messages. Efficient utilization of the PDCCH is the key for good LTE system performance and high capacity.
Specifically, an eNodeB as the LTE base station has to signal on the PDCCH per TTI to all scheduled UEs the corresponding allocation for UL and DL. In addition, also broadcast, paging and other common signaling is transmitted. In order to comply with these tasks, the PDCCH is partitioned into a common search space (CSS) and a UE specific search space (USS). Every active UE in the cell listens to the PDCCH (excluding the configured DRX periods). Though, a UE listens only on specific search positions according to its hashing function, which relies on RNTI and a sub frame number and the aggregation selected for the message. An aggregation defines the code-rate selected for the message, which is derived from CQI/radio quality measurements such that typically a target of 1% BLER (TARGET_BLER) is maintained. Unfortunately, the higher the aggregation, the lower is the number of potential search positions on the PDCCH. There are aggregations AGG1 (QPSK-2/3), AGG2 (QPSK 1/3), AGG4 (QPSK-1/6) and AGG8 (QPSK-1/12) possible with six potential search positions, six potential search positions, two potential search positions, and two potential search positions, respectively, on PDCCH available. Moreover, a high aggregation occupies more capacity on PDCCH, i.e. the terminal might suffer from higher blocking probability due to PDCCH hashing. Furthermore, depending on the message size, every aggregation level has a certain SNIR requirement for achieving the TARGET_BLER, i.e. a large MIMO message (e.g. DCI format 2/2a) requires higher SNIR than a small conventional message (e.g. DCI format 0, 1, 1a, . . . , 1d).
Thus, it is apparent that having several UEs allocated in a cell with different aggregations leads to a high probability that collisions occur, i.e. the colliding UE cannot be served in that particular TTI if the required search position on the PDCCH is already occupied by another UE. Such UEs may be termed as blocked. Moreover, it can happen that already scheduled UEs cannot be served due to collisions which are caused if the scheduler of a eNodeB does not take into account the PDCCH hashing, when selecting the UEs, i.e. the scheduling information for a specific UE is arranged in a search position, which has already been occupied by another UE. This leads to loss on the air interface by unused resources.
PDCCH also supports power control. By means of power relocation from one DCI message to another one a fine tuning of the target BLER can be achieved within certain power ranges of +/−4 to 5 dB as long as the maximum output power constraint of the eNB is maintained.
The total PDCCH capacity in terms of channel coding elements (CCEs) depends on the number of OFDM symbols reserved per TTI. For example, in 10 MHz bandwidth 1, 2 or 3 OFDM symbols can be allocated in a TTI for PDCCH.